Extending the 7 train to the Javits Center could cost nearly $1 billion more than expected, which could jeopardize other MTA expansion projects such as the Second Avenue Subway, Assemblyman Richard Brodsky said yesterday.

The city agreed to pay for the $2.1 billion project last year under a deal for development rights of the West Side yards, but is under no obligation to foot the bill for additional costs.

MTA head Elliot Sander said “the project is not on hold,” but that he would not award any construction contracts until the deal is renegotiated.

“I’m hoping we can work through it,” Sander said, adding he is concerned about endangering other capital projects including East Side Access into Grand Central, Second Avenue Subway and repairs to the system.

The extension is a pet project of Mayor Bloomberg and Deputy Mayor Dan Doctoroff, but never was part of the MTA’s capital plans.

The original price tag did not take into account rising construction costs, which Brodsky estimates already are between $100 million and $200 million higher, as well as the $100 million to $200 million the MTA would need to spend on new subway cars.

In order to keep the costs at $2.1 billion, several cuts were made to the project, such as delaying construction of a station at 10th Avenue.

That station will cost another $250 million to $500 million depending on when it is built, officials said.